1. Field of the Invention
The present invention relates to a zoom lens system and an electronic image pickup apparatus using the same.
2. Description of the Related Art
In recent years, it has been general to mount a zoom lens system as a photographing lens on an electronic image pickup apparatus such as a digital camera. Moreover, to miniaturize an electronic image pickup apparatus such as the digital camera, there has further risen a demand for thinning of the zoom lens system as the photographing lens.
However, in the zoom lens system for use in the digital camera including a small image sensor, a focal length shortens. Therefore, in a case where various aberrations are designed to be small while satisfying an oblique incidence characteristic of the image sensor, the number of lenses constituting the zoom lens system easily increases. Therefore, it becomes difficult to constitute the zoom lens system to be thin.
To solve the problem, in general, a thickness of the zoom lens system when stored (collapsed) is reduced by decreasing the number of the lenses constituting the zoom lens system including a plurality of lens units. It is considered that a zoom lens system including a minimum of two lens units be adopted in order to reduce a total length of the above zoom lens system. As the zoom lens system having its thickness reduced when stored, there is known a system described in, for example, Japanese Patent Application Laid-Open No. 2004-102,211.
The zoom lens system described in Japanese Patent Application Laid-Open No. 2004-102211 includes, in order from an object side, a lens unit A having a negative refractive power, and a lens unit B having a positive refractive power. The lens unit B is constituted of three meniscus lenses convex toward the object side, thereby reducing the total length of the zoom lens system.
Moreover, with a progress of high definition of the image sensor, an image is trimmed and enlarged so that it is possible to obtain an effect similar to that produced during zooming on a telephoto side. Therefore, there is a strong demand for a wide angle of the zoom lens system in addition to the reduction of the thickness of the zoom lens system.
A negative-lead type zoom lens system constituted of two or three lens units is advantageous in the reduction of the thickness. However, in a case where the number of the lenses constituting the lens units is large, a thickness of a lens element is large, or a lens closest to the object side is a positive lens, the thickness of the zoom lens system does not decrease even when the system is of a collapsible type. Such tendency is seen in zoom lens systems described in Japanese Patent Application Laid-Open Nos. 11-52,246 and 11-142,734. As an example of a presently known system which is suitable for the electronic image pickup apparatus and which is satisfactory in image forming performances such as a zooming ratio, an angle of field and the F number and which might be most thinned when collapsed, there is a system described in Japanese Patent Application Laid-Open No. 9-33810. When an entrance pupil position of a first lens unit is shallow, a diameter of the pupil decreases. As a result, the first lens unit itself can be thinned. However, for this purpose, a magnification of a second lens unit has to be high, that is, a refractive power of the unit has to be strengthened. Therefore, the image forming performance has to be sacrificed. Alternatively, the number of the constituting lenses has to be increased to thereby sacrifice reduction of depth.